netdev - Re: [PATCH net-2.6.24] introduce MAC_FMT/MAC

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Message-Id: <20070828.155419.74563234.davem@davemloft.net>
Date:	Tue, 28 Aug 2007 15:54:19 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mb@...sch.de
Cc:	joe@...ches.com, johannes@...solutions.net, netdev@...r.kernel.org
Subject: Re: [PATCH net-2.6.24] introduce MAC_FMT/MAC_ARG

From: Michael Buesch <mb@...sch.de>
Date: Tue, 28 Aug 2007 16:48:44 +0200

> On Monday 27 August 2007 23:11:50 David Miller wrote:
> > From: Joe Perches <joe@...ches.com>
> > Date: Mon, 27 Aug 2007 13:57:42 -0700
> > 
> > > On Mon, 2007-08-27 at 13:41 -0700, David Miller wrote:
> > > > From: Johannes Berg <johannes@...solutions.net>
> > > > Date: Mon, 27 Aug 2007 12:54:09 +0200
> > > > > #define MAC_FMT "%s"
> > > > > #define MAC_ARG(a) ({char __buf[18]; print_mac(a, __buf); __buf;})
> > > 
> > > > I don't think this works.
> > > 
> > > $ cat test_fmt.c
> > > #include <stdio.h>
> > > #include <stdlib.h>
> > 
> > You're just getting lucky in this test case.
> > 
> > The language does not allow what you are doing, so you're
> > playing with fire.
> 
> What exactly to you think it invalid in this code?
> I think it's fine (except that I'd chose an u8* for the mac
> address (first arg in print_mac()).

The __buf[] is used out of scope, therefore it's stack space is
fair game for the compiler to reuse.

When the compiler sees:

	printk(FMT, ({ char __buf[x]; print_mac(a, __buf); __buf;}));

It first all of the printk() argument expressions, first FMT and
then it evaluates the ({ ... }) argument.

Now that the ({ ... }) expression is done, __buf[] is out of
scope and illegal to reference.

printk() is now called, with a pointer to an out-of-scope buffer.
This is illegal.

I don't know how else to explain this to you, I can learn how to
describe the issue in German if this would help :-)
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